Flash evaporator



July 12, 1960 A. FRANKEL FLASH EVAPORATOR Filed April 2, 1957 I nvenlor F gttorney} t d g This invention relates toevaporators.

liquid is evaporated. ;The heated liquid is passed from flash chamber to flash chamber through orifices joining I the lower parts of the flash chambers which are arranged vertically side by side, and the .vapour passes upwards and out of each flash chamber through an outletlocated initsupperpart. p

It is a feature of such evaporators that, ,during normal operation, the pressure and corresponding saturation tem perature within the. flash chambers. decreases from flash chamber to flash chamber, audit isihighly desirable from the point of view of eflicient operation of theevaporator that there should beno leakageof vapour between adjacent flash chambers through the orificesq provided for the passage of the liquid beingevaporated. 3

Such leakage can be avoided by so dimensioning the orifices that the liquid level required to pass the circulated quantity of liquid from one flash chamber to the next covers the upstream side of the orificejoining the two. This liquid level varies however with variations the 10mm; the evaporator. Thus, on full load operation, the head forcing-the-liquidthrough-the orifice consists-of the static head difference between the liquid levels in the two adjacent flash chambers plusthe head resulting from the vapour pressure difference withinthe two chambers.

T Adolf Frankel, Gosforth, Newcastle-on-T yne, England, 5

Inmulti-stageflash-typeevaporators, it-is usuallto pro- T 'videa"series oi flash chambers whereinpart of the heated In contrast, on no load or low load operation, there is V substantially no vapour pressure difference from flash chamber to flash chamber, and the pressure drop required to force the liquid through the orifice has to be supplied solely by the difference in static liquid levels upstream and downstream of the orifice.

Generally, the orifices connecting the flash chamber are dimensioned so as to suit full load operation. This has the disadvantage however, that on no load or low load operation (for example, when starting up the evaporator), the difierence inthe liquid levels from' flash chamber to flash chamber increases by the amount of vapour pressure head which is available on full load operationbut which is'now non-existent. As this effect takes place in each stage, it is cumulative and the liquid level in the high pressure flash chambers 'may'rise to such an extent that excessive carry-over of liquid into the distillate takes place or even that some of the liquid spills over from the flash chambers into the associated heaters and contaminates the distillate.

According to the invention, the lower portion of each chamber is provided with a weir 20 or the equivalent the top of which lies above the openings in the chamber through which heated liquid enters and leaves the chamber. The provision of these weirs in the flashchambers prevents an excessive rise ofliquid level in the flash chambers on no load or low load operation. 'At the same time, the weirs prevent a drop in liquid livel on full load operation to an extent which would otherwise allow vapour to escape firom flash chamber to flash chamber.

lowerportions'of .two of'the'flash chambersf The apparatusshown in Figurel comprises a numher of rectangular flash chambers 10 arranged vertically Heated liquid which is to :be partially evaporated. in'the chambers enters thefirst chamber through thepipe 12 and passes from chamber to chamber through orifices' 14 in the lower parts of the'walls 16.

. i Patented July 12, 1960 'itwill now be described with reference to the accompanying drawings, in which:

.Figure 1 he vertical section through a number of flash chambers forming part of a flash-type evaporator;

Figure 2 is aperspective'view, with parts broken away,

of'one of the flash chambers shown in Figure l; and

Figure 3 isa sectionsimilar to Figure 1 showing side by side.

Outlets-18 are provided in the upper parts of each cham-' her through whichvapour is passed outof the chambers.

-As shown in Figures 1 and 2, the lower part of each.

flash chamber is provided witha weir 20 in the form of a plate which extends across the chamber and is connected thereto at its ends. The tops of these weirs lie above the orifices 14 and serve to control the liquid level in the chambers 10 in the following manner.

For any given operating condition, the liquid level upstream of each orifice 14 adjusts itself so that the difference in static head'across the orifice plus the vapour pressure 'diflerence on either side 'of'the wall 16 are pressure diiference available at full 'load between two adjoining flash chambers 10. Under. such conditions it can be arrangedthatl the static liquid level downstream of the orifice 14 is actually higher than the static liquid 1 level upstream, bythelarnountby whichlthesaid vapour pressure difierence is larger than the pressure drop across the orifice. Part of the total head available is then destroyed by the liquid spilling over the top of the weir 20 i.e. quite independently of the energy dissipation in the orifice. This additional energy dissipation in the weir means that the orifice has to be designed larger, i.e. to give a smaller pressure drop than would otherwise be needed, but the weir ensures that the liquid level in the upstream stage still covers the orifice entirely and prevents carry-over of vapour from stage to stage.

Under part load or no load conditions, -'when the pressure diflerence from stage to stage is small or abas the liquid level upstream of an orifice 14 has risen above the top of the weir 20 located in the corresponding flash chamber, the energy dissipation across the weir ceases and the whole of the static head difierence be,-

comes" available to pass'the circulated liquid through the orifice 14, which'is now designed to give a smaller pressure drop, as described above. The liquid levels in the chainbers on no load operation are shown at 22 in Figure 1. This ensures that the rise in the levels of the liquid in the flash chambers on no load or low load operation is kept well within permissible limits, and is in any case much smaller than what would be possible without the weirs. The liquid levels in the chambers on full load operation are shown at 24 and 26 in Figure 1.

As will be noted from Figure 3, the weirs 20 are arranged close to the chamber walls 30 in such a way that secondary orifices 32 are formed at or near the top of the weirs. It is of course possible, and may in some 7 discharged through the secondary. orifices 32,- andthis.

causes the heated liquid entering each chamber to bev projected upwardsthrough the secondary orifices, which improves the evaporation. Moreover, when the'full pres-' sure diiference is available across the flash chambers but the quantity of brine circulated through the system is reduced below the: full load' quantity, a certain amount of vapour may enter the. space'inside theweirs, i.e. be-. tween. the primary orifices 14 and; thesecondar-yorifices 32,v Such vapourcven. in .very. small quantities, considerably decreases the. density of the liquidevapour mixture discharged through thesecondary orifice 32, and this effect, for a given mass flow through theorifice, increases the pressure drop required .to project the mixture through the orifice.

The arrangement thus .has a stabilising effect on the flow. of the heated brine from chamber to chamber, because any vapour loss across the. primary orifice Mcauses a considerable increase in the pressure drop required to project a. given mass flow across the secondary orifice 32, and this in turn necessitates a rise in the static liquid level ahead of the primary orifice 14. to produce the necessary increase in pressure drop. This last effect, in turn, reduces the tendency for increased vapour loss acrossthe primary orifices. 14 with reduced circulation, while maintaining all the advantages described above regarding the prevention of flooding. when the full brine quantity is circulated but there is no pressure drop across the flash chambers.

As already mentioned, the secondary. orifices 32 are smaller than the primary orifices 14. Preferably, the cross-sectional area of the orifices '14 is at least twice that of the orifices 32 so that the pressure drop across the orifice is reduced when brine is circulated through them on starting up the evaporator, i.e. when there is. no vapour pressure difference between the flash chambers.

Baflle plates 34 are arranged above the orifices 32 in such a way that the liquid is projected on to them on leaving the secondary orifices. The .bafile plates prevent the liquid from being thrown up in large quantities to excessive heights, and they also assist the evaporation by dispersing the particles of liquid.

I claim:

In a flash evaporator, a rectangular fluid-evaporating section defined by apairof flat. side.,walls, a pair of fiat end walls, one of said end walls constituting an ,up-

am end W a aUOP w lll ilsif .fla b g n al fiat generally rectangular partitionwalls in, saidsection dividing said section into at least three adjacent rectangular flash chambers arranged in a straight line horizontally andcontiguously, said partition walls each ex tending from one of said sidewalls to the other of said side walls and from said top wall to said bottom wall, a simple aperture in the. lower portion. of each one of said partition walls providing communication between adjacent pairs of said flash chambers, and a similar aperture in the lower end ofeach end' Wall, a plurality of simple outlet apertures in the upper portion ofsaid-flashflphatm:

bers respectively, said apertures being in-one of aid-side' walls of said section, said last-mentionedapertures being located one each in the respective'fiash chambers, and;

serving as vapor outlets, a plate-like weir member arranged in each of the lower portions of saidfiash'chambers, saidmembers being disposed adjacent to and downstream of the said apertures in the lower portions ofsaidupstream end wall" and said partitions, each weir member comprising aflat vertical lower portion and an -inclined upper portion, said upper portion being inclined in upstream direction, bafile means fixed above said -weirmember to that wall of each chamberwhich-is upstream of saidxweir member, saidbafile means being in theform of a flat plate-like member extending from saidlast-mentioned wall of said flash chamberat'rightanglescthereto.

References .Cited in the file of this patent UNITED STATES PATENTS 1, 23 ter. Man 10. 1 8 2 693 350v Ne -2,. 1 5.4 2,759,882 

